I quite like the majority of the level design, the exception being that awful Green Hills act 3! You can spot most of the spike pits throughout the game (except Green Hills act 3!) by holding down when standing next to them - though I agree it does slow the pace down. However the 8 bit Sonic 1 and 2, and the 16 bit Sonic 1 all suffer the same thing - go fast and you hit something, so speed is only possible after memorising the levels. It's a fair criticism but not unique to 8 bit Sonic 2.
Overall I generally have more fun with this one because the level design is more open to speed running once you've learned the stages, whereas Sonic 1 is more restrictive and broadly the same pace on repeated playthroughs as your first few times.
Sonic 1 is definitely the more challenging game but I think you're being a little unfair on the boss fights for Sonic 2. Sure that first boss is easy - but remember Sonic 1 and 16 bit Sonic 2 also have insultingly easy opening boss fights. The others I think are much tougher, thanks to one hit deaths:
Stage 2 - an attack can easily rebound you into a fireball and the way the boss height varies means you have to time your attacks well
Stage 3 - narrow window between ball being ready to be burst and being launched, so you can easily mistime and be killed
Stage 4 - random attacks, don't be tempted to pre-empt one because you'll get caught out. As the fight goes on, the window for hitting the boss drastically reduces. Get it wrong and you die
Stage 5 - watch the timing of his spikes coming back out and mind those falling rocks
Stage 6 - very easy to get bounced around, confused, and end up squashed under him. Also getting wiped out when you're not rolled up into a ball
Stage 7 - did that fireball clear the screen? Nope it just swung back and killed you. A clear opening? Nope, he's fired those electrical things across the floor. Okay now it's definitely clear, right? Nope, launches fireball as you exit the tube
Ultimately the game has respawning extra life boxes and is quite generous with them in general, so you end up with enough lives that most difficulty spikes are trivial. Sonic 1 is much better balanced in that respect.